You are answering a USMLE-style medical question. Question: A 69-year-old man presents to his primary care physician for trouble sleeping. The patient states that he recently retired from working the day shift at a cemetery. When the patient retired, his goal was to finally be able to go out with his wife; however, he finds that he is unable to stay awake past 6 pm in the evening. His inability to stay awake has been straining his marriage as his wife is disappointed that they cannot do any activities in the evening together. The patient has tried drinking caffeine but finds that it does not help. The patient’s wife claims that the patient seems to sleep peacefully, and the patient states he feels rested when he awakes. The patient has a past medical history of irritable bowel syndrome which is managed with fiber supplements. The patient’s neurological exam is within normal limits. Which of the following is the most likely diagnosis? Options: A) Obstructive sleep apnea B) Advanced sleep phase disorder C) Chronic insomnia D) Normal aging Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.